Seismic holography

ABSTRACT

By utilization of the principles of holography, a remote object, usually a seismic anomaly, is made visible and subject to detail examination by visual or optical means. Coherent acoustical energy is transmitted into the earth or other elastic medium and the energy received with areal detector means. A reference signal obtained from the energy source is delayed, amplified, and mixed with the signals from the areal detector means to obtain a conventional holographic interference pattern. Scaling is accomplished by an optical reduction. A seismic model is used to provide an ultrasonic test signal equivalent in major respects to seismic field conditions for recording acoustical holograms.

United States Patent Riggs [541 SEISMIC HOLOGRAPHY L72. Inventor: EmmetD. Riggs, Dallas, Tex. 75208 [73] Assignee: Atlantic Richfield Company,New

, York,N.Y. 7 v i [22] Filed: July 25, 1969 [21] Appl. No.: 844,864

Thurstone, Holographic Imaging with Ultrasound Journal of AcousticalSoc. of America, 4-1969, pp. 895- 899,

Bakhrakh, Use of Holography in Reconstruction of Polar Diagram of UHFAntennas from Field Meas. in

LASER 3| DIFFUSER TRANSMITTER 1 Sept. 12, 1972 the Fresnel Zone SovietPhysics- Doklady, Vol. 1 1, No. 12, 6- 1967, pp. 1102-1104 Korpel,Visualization of the Cross Section of a Sound Beam by Bragg Diffractionof Light" Applied Physics Letters, Dec. 1966, pp. 425- 427 PrimaryExaminer-Benjamin A. Borchelt Assistant ExaminerS. C. BuczinskiAttorney-Blueber S. Tharp and Robert E. Lee, Jr.

[57] ABSTRACT By utilization of the principles of holography, a remoteobject, usually a seismic anomaly, is made visible and subject to detailexamination by visual or optical means. Coherent acoustical energy istransmitted into the earth or other elastic medium and the energyreceived with areal detector means. A reference signal obtained from theenergy source is delayed, amplified, and mixed with the signals from theareal detector means to obtain a conventional holographic interferencepattern. Scaling is accomplished by an optical reduction. A seismicmodel is used to provide an ultrasonic test signal equivalent in majorrespects to seismic field conditions for recording acoustical holograms.

3 Claims, 5 Drawing Figures MEANS PATENTED EP 2 I972 SHEET 1 BF 3' mm a23. //\y@ mm F By RMWM ATTORNEY PATENTEDsEmmn 3.691.517 sum 2 or a zooo'LIGHT FIBER- INVENTOR.

Emmet D. Riggs A 7' TORNE Y SEISMIC HOLOGRAPHY BACKGROUND OF THEINVENTION The invention pertains to exploration Seismology andparticularly concerns the application of the principles of holography toseismic prospecting. The same techniques, however, can be extended tothe examination of three-dimensional objects of any sort by means ofcoherent elastic waves in any medium in which they can be propagated.

General teachings of interest concerning holography and its applicationinclude the following material which is introduced herein by reference:

Leith and Upatnicks, Scientific American, June 1965, in an articleentitled Photography by Laser."

Pennington, Scientific American, February 1968, in an article entitledAdvances in Holography.

El-Sum, Science and Technology, November 1967, in an article entitledUses for Holograms.

Ennos, Contemporary Physics, Vol. VII], No. 2, 1967,

in an article entitled Holography and Its Applicatrons."

Collier, IEEE Spectrum, July 1966, in an article entitled Some CurrentViews on Holography.

Thurstone, Proceedings of the IEEE, April 1968, in an article entitledOn Holographic Imaging with Long Wavelength Fields.

Metherell, El-Sum, Larmore, Acoustical Holography, Vol. 1, Plenum Press,New York, 1969, in a series of articles presented at the Proceedings ofthe First International Symposium on Acoustical Holography held atHuntington Beach, California, December 14 and 15, 1967.

Goodman, Introduction to Fourier Optics, McGraw- Hill Book Company, NewYork, 1968, in Chapter 8, entitled Wavefront Reconstruction Imaging orHolography."

Holography is, in essence, a wavefront reconstruction process. When adiffusely reflecting object is illuminated, the reflected illuminationfrom'each discrete point on its surface forms a system of expandingspherical waves producing a complex irregular wavefront that containsinformation about the object. When a suitable reference wave is combinedwith coherent illumination from the object, the resulting recording,i.e. fringe pattern, is called a hologram. This recording has a uniqueproperty that when it is subsequently illuminated with a suitablecoherent reference wave, the original wavefront is re-created on passagethrough the hologram to form a wavefront similar in most respects tothat from the original object. Since the reference wave produces aninterference pattern in which the original wavefront is reconstructed,an observer, viewing the hologram, sees an illusion which is a replicaof the original object.

A hologram is thus a recording of an interference pattern between twocoherent fields. One is from the illuminated object and the second isthe reference wave. The requirement of the recording medium is that itmust be capable of resolving the minimum wavelength of the interferenceproduced by the addition of these two fields. In an acoustic system thewavelength of the energy radiated from the illuminated object may varyfrom the ultrasonic to those experienced in geophysical seismicoperations. Since the final hologram must be examined under illuminationin visible light range,

there is a scaling factor of considerable magnitude involved. Scalingcan be done in a number of ways and the amount of scaling is primarilydependent upon the wavelength of the original field used to illuminatethe object and serve as the reference wave. If the reference wave usedto record the hologram and the coherent wave used in the reconstructionprocess are both plane wavefronts, the hologram must be scaled by theirwavelength ratio in order to reconstruct an image the same size of theoriginal object. In the case of geophysical applications, a reduction bythis ratio generally introduces an image reduction which requiresoptical magnification for satisfactory observation.

At least two patents directed toward seismic holography have alreadyappeared. Reference is made to Silverman, U. S. Pat. No. 3,400,363,patented Sept. 3, 1968, and Silverman, U. S. Pat. No. 3,450,225,patented June 17, 1969. Silverman proposes to transmit continuous soundwaves of constant frequency into the earth and detect the returnedenergy with areal detector means. Silvermans detector means is comprisedof a plurality of transducers forming a grid of points over the detectorarea connected to a plurality of light emitters (glow lamps). Eachtransducer controls illumination of one of the light emitters accordingto the intensity of sonic energy received at its position. However, itis first necessary to mix each received signal with a reference signal,i.e. the transmitted sound waves. In the first cited patent, thereference signal is obtained from the sound source. In the second, thereference signal is derived from a summation process based on thereceived signal. It is stated that by arranging the array of lightemitters in the same pattern as the detector transducers, a photographicrecord can be obtained which is the equivalent of a hologram.

However, there are still many problems that must be solved beforeseismic holography becomes a practical tool. One point of difficulty ishow to solve the problem of specular reflections. Thus, under usualconditions an image of the energy source is obtained rather than animage of the object being illuminated. Another point of difficulty froma practical standpoint is the large number of seismometer stations whichmust be laid out on the surface of the earth in order to sample asufficient number of points to record a hologram. Still another problemis that of scaling so that a hologram recorded at acoustical wavelengthscan be reconstructed with coherent light. Another problem has been theneed for a seismic model whereby acoustical holography can be performedunder controlled conditions in the laboratory.

SUMMARY OF THE INVENTION The present invention offers solutions to theabove stated problems and points of difficulty found in seismicholography. Applicant has invented (l) a process for eliminatingspecular reflections of the energy source, (2) a process for fieldrecording holograms using a reduced number of seismometer stations, (3)a process for scaling acoustical holograms so that they can bereconstructed with light, and (4) a seismic model whereby acousticalholograms can be recorded using ultrasonics.

According to conventional seismic holography procedures, coherentacoustical energy is transmitted into the earth from an energy source onthe surface and subsurface reflections thereof are received with arealdetector means at a plurality of seismometer stations. The energy sourceis a continuous wave generator such as an electric shaker, an hydraulicpiston or the like, capable of generating sonic waves of controlledfrequency. The detector means forms a two-dimensional array over apredetermined area of the earth's surface and is comprised of a largenumber of seismometer stations (each which may have one or moreseismometers), preferably arranged in a regular geometrical fashion.

One aspect of Applicants invention deals with the problem of eliminatingspecular reflections of the energy source so that reconstruction willshow an image of the subsurface object being inspected. This is done byusing a diffuser transmitter as the energy source. Instead oftransmitting acoustical energy into the earth at a single point (orsmall area), the diffuser transmitter synchronously transmits energy ata plurality of spaced apart points. When the output of the energy sourceis diffused in this manner, undesirable specular reflections areeliminated.

Another aspect of the invention relates to a process whereby the numberof seismometer stations in the detector means can be minimized. Inbrief, this is done by determining or calculating what seismic responseswould be obtained at locations intermediate (between) the actualseismometer stations. Responses obtained at seismometer stations oneither side of the location in question are delayed, selectivelyweighted, and combined according to a predetermined function in order topredict the response at the location. Obviously, if one can accuratelyinterpolate what the seismic response will be at a given location, it isredundant to locate an actual field seismometer station at the location.(Rules for establishing a maximum spacing between the seismometerstations are discussed in the Preferred Embodiments") Another aspect ofthe invention which is considered extremely important by Applicant ishis analog process for solving the scaling problem. According to theprior art, seismic signals from each of the seismometer stationscomprising the detector means are combined with a reference signal toproduce corresponding interference signals. An array of light sources isformed such that their geometrical relation is the same as that of theseismometer stations and each seismometer station is represented by oneof the light sources. The luminous intensity of each of the lightsources is modulated by the appropriate interference signal and aphotographic exposure is made to form a hologram.

Applicants improvement comprises scaling the distance between the lightsources relative to the distances between the seismometer stations(actual or interpolated) by a factor equal to the ratio of thewavelength of light to be used for reconstruction to the wavelength ofthe reference signal. Since reconstruction is with light whereas thereference signal is an acoustical wave, the light sources must be pointsources packed in a matrix at very close spacing. Light fibers having acenter to center spacing on the order of one micron are suitable sourcesfor this purpose.

The final aspect of the present invention concerns a seismic model forstudying acoustical holography in the laboratory. It comprises a testchamber having ultrasonic transducer means and directional microphonemeans attached near opposite ends thereof. A target anomaly is scaled toultrasonic proportions and placed on the floor of the chamber. Inoperation, coherent acoustical energy is transmitted toward the targetanomaly and reflections are received by the directional microphones. Areference signal is mixed with each of the received signals and scalingis accomplished as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing showingApplicants apparatus for seismic holography illustrating the componentsrequired for the recording of a seismic hologram.

FIG. 2 illustrates a method of viewing a hologram obtained from theapparatus shown in FIG. 1.

FIG. 3 is an illustration of the arrangement of detector means on theground for the recording of an acoustical hologram by seismic means.

FIG. 4 shows the arrangement of the output ends of the optical fibersarranged in a matrix forming the scaler.

FIG. 5 is an illustration of the ultrasonic signal generator and seismicmodel for recording acoustical holograms.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) To gain a betterunderstanding of the invention, refer first to FIG. 1 which shows howseismic holograms are recorded. Oscillator 10 is the frequencycontrolling portion of the energy generating system. In conventionalfield operations this frequency controlling source may be a preciselyregulated electronic oscillator operating in the seismic band or it maybe a prerecorded trace which is being played back. Control means 11provides necessary phase control or delay among the signals provided todiffuser transmitter means 12. Coherent acoustical signals aretransmitted by diffuser transmitter means 12 according to the outputsignal of oscillator 10. Diffuser transmitter means 12 may be acontinuous wave generator such as an electric shaker, an hydraulicpiston, or the like, capable of producing a carefully controlledacoustical equivalent of the oscillator signal. This transmitterdiffuser means has multi-element units 13 randomly or regularly spacedto cover an areal extent.

One of the requirements of diffuser transmitter means 12 is that itgenerate an acoustical wave in the earth, having a maximum energy lobedirected in the direction of the anomaly of interest. This requirementcan be met by the introduction of an appropriate phase delay entered bycontrol means 11 dependent upon the geometrical location of each elementof the diffuser transmitter in operation. The second requirement is thatthe actual wavefront, consisting of energy from units 13, contain slightphase differences such that the effect of the overall energy pattern isto produce a diffuse type of reflection from the anomaly which is beingilluminated.

Diffuse illumination is desirable instead of specular because theresulting visual observation of the final hologram produces an image inwhich the anomaly of interest can be viewed in a normal fashion and allparts observed from one position of observation. In contrast, if theobject being mapped is illuminated by a non-diffuse type of illuminationso that specular reflections are obtained, the object can be seen onlyas a reflection from the source supplying the acoustical energy. Hence,diffuse type of illumination produces a much more usable hologrambecause the objects appear as the observer expects from experience tosee an object viewed optically.

In practice, the weathered layer normally associated with the nearsurface of the earth has been found to be useful in producing a diffusetype of reflection. The weathering of the near surface from the actionof water, wind, and temperature variations produce uneven acousticalconditions near the surface. If the appropriate phase delay isintroduced to form the output of units 13 of diffuser transmitter means12 into a directed beam with the main energy lobe oriented towards thesurface of the anomaly of interest, then the slight delay imposed by thevariations of near surface weathering serves to produce a semi-diffuseenergy source having an average plane wavefront. By using a sufficientnumber of units 13, a totally diffuse energy output can be obtained andspecular reflections completely eliminated.

Referring further to FIG. 1, the reflected energy from subsurfaceformation 14 is detected by seismometer stations 15 and the resultingseismic signals are modified in a manner indicated subsequently. Eachseismometer station 15 may consist of one or a plurality of seismometersconnected to a common output. As will be seen in conjunction with FIG.2, seismometer stations 15 in reality form an areal detector meanscovering a preselected area or region of the earth. It is understood,however, that only one seismometer station is actually needed since itcan be moved from location to location while diffuser transmitter means12 transmits repeated signals so that the final result is arealcoverage.

In FIG. 1, three seismometer stations 15 have been shown; however, itwill be seen that there are five seismic output channels. What Applicanthas done is to use delay units 16, multiplier means 17, and summationmeans 18 to interpolate seismic signals intermediate to the signalsrecorded at actual field seismometer stations. Delay units 16 provide adelay for the received seismic signals so that they are properly timealigned for normal moveout. Multiplier means 17 then weights the seismicsignals relative to each other according to a predetermined function(usually linear based on surface distance). Summation means 18 combinesor adds input signals from multiplier means 17 in order to determine theseismic signal at a preselected point intermediate to the signals beingoperated upon. If desired, several seismic signals may be interpolatedin this manner between adjacent pairs of seismometer stations 15.

Oscillator 10, in addition to exercising frequency control over thevarious elements of diffuser transmitter means 12, also provides areference signal which is fed to delay unit 20. Alternatively, therequisite reference signal is obtained by means of seismometer station19 located adjacent to diffuser transmitter means 12. In either event,the reference signal is tip propriately delayed by delay unit 20 andthen weighted or amplified by multiplier means 21 to a predeterminedlevel.

Delay unit 20 provides a delay for the reference signal so that theeffect produced at summation means 22 is that of a uniform planewavefront of the reference signal arriving at a predetermined optimumangle from the opposite direction to that of the emerging wavefront ofthe seismic signal. This is a conceptual device to avoid confusion withthe idea of the reference wave traveling through the weathered nearsurface. In reality, the reference wave is added in such a manner thatthere is a phase shift between the average value of the emerging seismicwave equivalent to a reference angle (usually 30) between the twowavefronts. Multiplier means 21 serves to change the signal or theimpedance level of the output of delay unit 20 and has uniform phasedelay so that the individual channels from delay 20 are altered on afixed and controlled manner necessary to adjust their level for properfunctioning relative to the seismic signals so that maximum interferencesignals can be formed. The reference signal is then separately mixedwith each of the seismic signals by summation means 22 in order to formcorresponding interference signals.

Amplifier means 23 are used to convert the summation signals fromsummation means 22 to voltages with impedance levels necessary to drivelight sources 24. Since'this is a steady state system, incandescent typelamps may be used for light sources 24 since the phase delay involved inany modulation scheme is not a factor. One type of lamp which has beenused is Chicago Miniature Lamp Works, Type Tl/2 No. CMA752 requiring 2volts at ma for normal illumination. This lamp has an overall maximumdiameter of 74/1000 of an inch. Another lamp suitable for this operationis manufactured by Los Angeles Miniature Products and is their lamp No.3 having a maximum diameter of 93/100 inch operating at 1.5 volts and 15ma for normal illumination.

Optically connected to the lamp means 24 are optical transformersections 25 which serve to couple lamps 24 to optical fibers 27.Technically, each optical section 25 is approximately 1 inch long andhas a maximum diameter sufficient to match or couple into one of thelamps 24. It is approximately one-tenth inch in diameter at the lamp endand about 25/1000 inch at the optical fiber end. The lamp end is formedinto a spherical shape to partially fit over the end of the lamp toprovide maximum coupling optimum of the light output and the opticalfiber end is drilled so that the optical fiber can be inserted andcemented in place. The other ends of the light fibers are arranged in anopaque holder (not shown) so that the individual lamps may be examinedor replaced if necessary. Likewise, for ease of handling, the individualtransformer sections are inserted into a plate or holder (not shown)such that the output optical fibers project through the opposite side.The transformer mounting plate and the lamp mounting plate are keyed sothat when they are placed together and fastened, the individual unitsare aligned in proper sequence.

The use of light fibers for the conduction of light is, of course, wellknown. Smooth fibers of transparent material such as glass conductslight with high efficiency because of total internal reflection alongthe walls with the result that individual fibers in a cluster or bundleconduct light independently of one another. In practice, there arepresent a number of minute defects and contaminations at the interfacewhich interfere with the total reflection by absorption scattering. Witha light fiber these losses become serious because there may literally behundreds or thousands of these minute imperfections as the light passesalong the fiber. Thus, an uncoated glass fiber is, in practice, a verypoor means of transmitting light. In addition, an uncoated fiber incontact with another uncoated fiber permits some leakage of light acrossthe interface. Both of these short-comings are largely overcome,however, by coating the fiber with glass or other material having adifferent index refraction from that of the fiber. In this particularcase, we are not interested in the highest efficiency of transmissionalong the fiber since only a short piece of fiber is used but rather weare concerned with the ability to reduce the fiber to a very smalltermination and to reduce the cross feed or leakage into adjacentchannels. In practice, this particular application has used a fiber witha core having an index refraction of 1.62 and coated with a glass havinga 1.52 index. Such fibers are available from the American OpticalCompany and can be obtained in a number of sizes. The 75 to 100 microndiameter fibers are convenient to manipulate and are useful in thisparticular fabrication. They are easy to manipulate and can be heatedand stretched until their diameters are sufficiently reduced.

The face plate 26 serves as a termination for optical fibers 14 and isfabricated on a special fixture having provisions for microscopicmanipulation of the fiber and optical observation at each stage of theobservation. In practice, each fiber 27 is cemented into transformersection 25 by use of an optical cement such as Canadian Balsam orpreferably a synthetic material used to cement elements of lensestogether and the assem bly is mounted in the fixture previously referredto. The end portion of optical fiber 27 is heated under controlconditions, stretched until its cross section diameter is 1 to I /zmicrons in dimension, placed in its appropriate position in face plate26 and cemented into position. After the complete operation of mountingthe entire group of fibers, the fixture is removed and face plate 26 isoptically finished by methods well known in optical technology.

Face plate 26 serves as an object for photographic lens 28 which formsan image on photographic means 29. This photographic means is normallyan Eastman Kodak S.O. 649F photographic plate. This plate is securedinto position on a transport means 30 so that as the detector meansindicated as seismometer stations 19 is moved, transport means 30 isadjusted to a new position so that the image formed by lens 28 fallsupon the appropriate position on photographic means 29.

Photographic film used in the linear range of its exposure logisticcurve is a square log detector and because of this squaring process itstores neither phase nor amplitude but instead a time averaged intensitydistribution of the light striking the film. In the hologram, the dropsand irregular interference patterns observed under a microscope on adeveloped plate represent an interference pattern between the referencewave and the light emitted by the object being recorded. Since we areconcerned with the photographic resolution obtainable in a hologram andvisible to the observer by optical means, the resolution capabilities ofthe various components of this system are critical.

After the recording of the data is accomplished on photographic means29, the plate is processed using conventional photographic techniquesand preferably followed by some well-known bleaching process to increaseits efficiency as a hologram. The resulting hologram is then viewed asindicated in FIG. 2. In this conventional method of viewing a hologram,laser 31 provides a coherent output in the visible range. This output isfocused to a point by lens 32, normally a very short focal lengthmicroscope objective. The output of the lens 32 is focused onto apinhole or aperture of the aperture plate 33. This aperture is made onthe order of 10 to 20 microns in diameter for optimum light transmissionwhile still obtaining maximum rejection of spurious modes from laser 31.Lens 34 is used to reform the beam into a source of coherent light of asize convenient to illuminate the hologram 35 for viewing. This lensnormally is a long focal length lens on the order of 20 to 40 cm focallength. Optical means 37 is normally required to enlarge the imageobserved in hologram 35 for visual observation by observer 36. If lasermeans 31 is a helium neon laser, the light available to observe thehologram 35 is illuminated with 6,328 angstrom wavelength light. Thefringe pattern generated will have a repetition rate of 790 linepairs/mm. Since Eastman Kodak plates, special order 649F, have aresolution in excess of 2,000 line pairs/mm, the limiting factor is theresolution capability of the lens used. There are a number ofcommercially available lenses which meet this criterion, one of which isa 30 mm F12 ultra micro lens from Ehrenreich Photo Optical Industries,Inc., Gordon City, NY. This lens will resolve in excess of 1,200lines/mm.

Applicant has discovered that the optically most efficient hologram isobtained if the exposure of the photographic means 17 of FIG. 1 iscarried beyond the linear range of the log exposure density curve of thephotographic emulsion being used if one of several processes are used toconvert a density hologram into a phase hologram based on the effectsobtained by variation in the index of refraction between variousportions of the recorded image pattern. This increased efficiency isbased upon the fact that in a bleached hologram the modification of thereference beam by the hologram is the result of variations in indexrefraction instead of variations in the density of the developedemulsion.

The apparatus illustrated in FIG. 2 indicates a viewing of hologram 35after the holographic recording has been accomplished. The hologram 35can be observed in real time if part of the apparatus of FIG. 2 iscombined with the proper photographic recording means 17 of FIG. 1 sothat the visual image recorded on photographic means 17 is observedwithout subsequent chemical development required for conventionalphotographic plates or film.

Referring to FIG. 3, typical detector means is illustrated comprised ofseismometer stations 15 on foot spacings and having a total arealcoverage of 2,000 X 2,000 feet. The seismometer spacing of 100 feet isindicated in FIG. 3 as a typical value necessary for field operations.Since the smallest wavelength that we may deal with in field practicemay be of the order of 500 feet, this value indicates five samplingpoints per wavelength. In the typical values of a wavelength of 200 feetalong a ray path, it has been found that the surface sampling intervalof one-half wavelength provides reasonable quality of recoveredwaveform. For reasons already discussed, it is considered a goodpractice, but not always completely essential, to interpolate betweenthe individual sampling points provided by adjacent seismometer stations15. This interpolation means is provided in FIG. 1 by delay units 16 andmultiplier means 17 along with summationmeans 18. Although only one ofthese summation means that is associated to delay and multiplier meansis indicated between the individual seismometer channels, this does notrestrict the invention to one interpolation. The only requirement isthat the phase delay between the signal derived from adjacent channelsbe proportionately spaced so that the final recovered signal ofinterpolation points have some known functional relationship to theadjacent seismometer date points. This relationship has been found to benormally linear.

It is assumed that the frequency of oscillator is maintained preciselyat 40 cycles and the seismic velocity of the near surface immediatelyunder the weather layer is 8,000 feet per second, the wavelength of anemerging signal is of the order of 200 feet measured along a ray path.However, since this wave strikes the interface between the surface ofthe earth and the atmosphere above at which or near which the detectormeans composed of a multiplicity of seismometer stations is located, theeffected wavelength changes. In the case of a shallow emerging wave, theeffected wave along the surface is about 2,000 feet and the effectedwave for an emerging ray from depth is about 500 feet. This can be moreeasily understood if it is assumed that there is a steady statewavefront composed of many components being reflected from the anomaloussubsurface structure. These steady state waves are arriving at.;thesurface and are being detected by the matrix or grid of seismometersindicated as 15. The reference signal may be assumed to be added to thewaves arriving from the subsurface. ln ordinary seismic prospecting, theangle of incidence for the emerging wave (angle from the vertical) canbe assumed to be of the order of 5 to 20. To this emerging wave we mustadd a reference wave which can be assumed to approach the surface fromthe opposite direction. In practice it has been found that this angle,which is the sum of the angle of incidence of the emerging wave plus theangle of incidence of the reference wave, should be on the order of toproduce the maximum usable resolution of small detail from thesubsurface anomaly yet provide reasonable resolution requirements forthe optical components to follow in the equipment.

Referring to FIG. 4, termination plate 26 is shown having a matrix oflight fibers 27. In FIG. 3 the detector means has 20 seismometerstations in each direction for a total of 400. Therefore, at a minimum,termination plate 26 would contain 400 fibers. However, by means ofApplicants interpolation technique, it is readily possible and desirableto determinate three interpolation points between each adjacent pair ofseismic signals. By way of illustration, termination plate 26 would thenhave a matrix dimension of 77 X 77 fibers for a total of 5,929. Thephysical dimensions of termination plate are about 20 X 20 inches andthe light fibers are spaced approximately one micron apart (center tocenter).

Another unique feature of this invention is the apparatus whereby therealistic test signal can be generated for purposes of alignment or toreduce holographic images from an anomalous model. Referring to FIG. 5,the seismic model is contained within test chamber 39. Oscillator 40 isthe means of frequency control of the energy which is transmitted fromthe diffuser transmitter means 42. Control 41 serves as a delay meanswhereby the various elements 43 of the diffuser transmitter are shiftedin phase so that the average phase of the energy is directed towardsanomaly of interest 45 as indicated by the wavefront. Transducers 43which compose diffuser transmitter means 42 may consist ofelectromechanical transducers of the piezoelectric type suitable foroperating at the desired frequency. They may be barrium titanate,quartz, ADP, tourmaline, or other suitable materials. The entiretransmitter assembly is separated from its supports by low-passmechanical filter sections 44. The sonic energy generated by diffusertransmitter means 42 is reflected off of anomaly model 45 towardexponential horns 47 of directional microphones 48. Each microphone 46is isolated from the other microphones and suspended from its support bylow-pass mechanical filter section 48.

Frequency of operation of this test system depends upon the dimensions.If the velocity of sound and air is approximately 1,100 feet/second anda wavelength of one-half inch is convenient, then the system shouldoperate near 13 khz. The directivity indicated by both diffusertransmitter means 42 and receiving microphones 46 reduces both thedirect feed from transmitter elements 43 and spurious reflections fromthe walls, ceilings, and other reflecting surfaces in the immediatevicinity of the model. The requisite reference signal is obtained fromoscillator 40 and is introduced into the delay means 20 of FIG. I. In asimilar manner, the outputs of microphone detectors 46 are introducedinto delay means 16 of FIG. I. This acoustical model is capable ofproducing holograms as a means of alignment and adjustment of theapparatus shown in FIG. 1 with a signal which is free of the usualspurious noise encountered in field operations. In a similar manner, itis also useful to produce holograms from scaled anomalous models.

What is claimed is:

l. A process for seismic holography comprising a. transmitting acoherent seismic signal into the earth, receiving reflections at aplurality of detector stations positioned over a wide surface area,

. combining the reflected signals with a reference signal to producecorresponding interference signals,

. providing an array of light sources so that a light source can bemodulated by each of said interference signals,

e. reducing the outputs of said light sources to point sources by meansof capillary light fibers having a predetermined diameter at theirterminal end,

f. arranging the terminal ends of said light fibers in a matrix so thattheir center to center spacing is in accordance with the ratio of thewavelength of light which is to be used for reconstruction to thewavelength of said reference signal,

g. connecting said light fibers to said light sources so that theirterminal ends will be in the same geometrical relationship in respect tosaid interference signals as said detector stations, and

h. modulating said light sources with said interference signals, and

i. recording a hologram based on the outputs of said light fibers.

2. Apparatus for seismic holography comprising a. seismic signaltransmitter means,

b. areal seismic signal receiving means comprising a plurality ofhorizontally spaced detectors,

c. reference signal means,

d. summation means for combining the detected seismic signals with thereference signal,

e. a plurality of light sources arranged in the same geometrical orderas said detectors, each of said detectors being associated with one ofsaid light sources,

f. a fiber optical system connected to the outputs of said light sourcescomprised of individual light fibers having their terminal endscritically spaced from one another according to the ratio of thewavelength of light to be used for reconstruction to the wavelength ofsaid reference signal, and

g. photographic means for recording the outputs of said light sources toproduce a hologram.

3. Apparatus according to claim 2 where the terminal ends of adjacentlight fibers have center to center spacings on the order of one micron.

1. A process for seismic holography comprising a. transmitting acoherent seismic signal into the earth, b. receiving reflections at aplurality of detector stations positioned over a wide surface area, c.combining the reflected signals with a reference signal to producecorresponding interference signals, d. providing an array of lightsources so that a light source can be modulated by each of saidinterference signals, e. reducing the outputs of said light sources topoint sources by means of capillary light fibers having a predetermineddiameter at their terminal end, f. arranging the terminal ends of saidlight fibers in a matrix so that their center to center spacing is inaccordance with the ratio of the wavelength of light which is to be usedfor reconstruction to the wavelength of said reference signal, g.connecting said light fIbers to said light sources so that theirterminal ends will be in the same geometrical relationship in respect tosaid interference signals as said detector stations, and h. modulatingsaid light sources with said interference signals, and i. recording ahologram based on the outputs of said light fibers.
 2. Apparatus forseismic holography comprising a. seismic signal transmitter means, b.areal seismic signal receiving means comprising a plurality ofhorizontally spaced detectors, c. reference signal means, d. summationmeans for combining the detected seismic signals with the referencesignal, e. a plurality of light sources arranged in the same geometricalorder as said detectors, each of said detectors being associated withone of said light sources, f. a fiber optical system connected to theoutputs of said light sources comprised of individual light fibershaving their terminal ends critically spaced from one another accordingto the ratio of the wavelength of light to be used for reconstruction tothe wavelength of said reference signal, and g. photographic means forrecording the outputs of said light sources to produce a hologram. 3.Apparatus according to claim 2 where the terminal ends of adjacent lightfibers have center to center spacings on the order of one micron.